Urea is generally produced from ammonia and carbon dioxide. It can be prepared by introducing an ammonia excess together with carbon dioxide at a pressure between 12 and 40 MPa and at a temperature between 150° C. and 250° C. into a urea synthesis section. Typical urea production plants further comprise a recovery section and a finishing section. In the recovery section non-converted ammonia and carbon dioxide are recovered and recirculated to the synthesis section. In the finishing section, typically, a urea melt is brought into a desired solid, particulate form, generally involving techniques such as prilling, granulation, or pelletizing.
The present invention relates to the production of solid urea, i.e. to urea finishing, A background reference relating to urea finishing is WO 2013/055219. Therein a method is disclosed wherein urea crystals are produced by subjecting a urea solution to flash crystallization, and subsequently a shaping step wherein the urea crystals are subjected to mechanical force.
An end-use of urea that is of increasing practical importance, is in the form of an aqueous solution for the abatement of nitrous oxides formed in combustion processes. The solution is known as diesel exhaust fluid (DEF), available, e.g., under the trademark AdBlue®, and is typically used in vehicles with diesel engines to reduce NOx formed during the combustion of diesel fuel. The reduction typically takes place with the help of a catalyst at elevated temperatures. During the reaction, the urea decomposes in ammonia and CO2, whereby the ammonia reacts in turn with the nitrous oxides in the exhaust gas stream of the diesel engine to form nitrogen and water.
A urea solution for use as DEF needs to meet stringent requirements, in particular with respect of the level of by-products, such as biuret (max 0.3%) and ammonia, and additives used during the urea finishing process, such as formaldehyde. The by-products and additives are allowed only in minor amounts in the DEF solution. These requirements are laid down in international standards such as DIN V70070 and ISO 22241. These standards are identical with respect to the specification.
Several methods have been described to produce DEF solution, e.g. by diluting urea granulate or prills in water. Typical problems associated with diluting urea after finishing, is the relatively high level of biuret and the presence of additives, such as formaldehyde which is added to increase the strength of the particles and reduce caking (stickiness). With reference to the aforementioned standards, the additives typically need to be removed to make the solution suitable for use as DEF, which requires costly and energy intensive processes.
A background reference relating to a method that avoids the need for removing additives, is disclosed in EP1 856 038. Therein, a DEF solution is prepared by taking a urea comprising aqueous stream separated directly from or after a recovery section in a urea production process, and thereafter diluting with water until the urea comprising stream comprises 30-35 wt. % urea. This method advantageously provides a solution which is directly suitable as DEF. However, since the end-product is a solution, its distribution in effect means that about two thirds water are transported, which is costly and which requires tank wagons instead of standard lorries.
Producing and handling a solution cannot be easily avoided. Urea is a product which is known to have properties which are not favorable for bulk transportation. In powder form urea is very sticky and is not free flowing. As the most common application of urea is that as fertilizer which is distributed over agricultural land, it is necessary to provide the final urea product in the form of a solution in water or as free flowing pellets which can be easily distributed. Two main processes are in use to produce these pellets. Firstly prilling, which involves contacting droplets of melted urea with a stream of cooling air in a prilling tower. The cooling air removes the heat of crystallization and the prills are collected at the bottom of the prilling tower. A disadvantage of prills is that the size of the pellets as well as the mechanical strength is limited and the prills tend to stick together during transportation over significant distances. This effect is called caking. A second process is fluid bed granulation which is successful in addressing some of these problems. The granules are typically larger than the prills and show a lower tendency to caking which makes them more suitable for transportation. Typically, additives such as formaldehyde are added to the urea melt before granulation to further improve the strength of the granulate and reduce the caking tendency. These additives, as referred to above, are undesirable, if not straightforwardly prohibitive, for using the resulting urea in DEF.
It is therefore desired to provide a urea product, suitable for the production of DEF, which has a sufficient purity of its own to meet the specifications for DEF, but which does not require transporting a great amount of water as of its production site. Particularly, it is desirable to have a free flowing urea powder for the preparation of a DEF solution.
In the art, no free flowing urea powder that can be transported to the desired location and subsequently be used to easily prepare a DEF solution, is as yet unknown.
It would further be desired to provide a urea powder that can be used to prepare a solution for treating flue gases from industrial furnaces such as reformers. Herein a thermal rather than a catalytic process can be used, and the applied solutions are generally more concentrated than DEF. It is generally difficult to transport a concentrated urea solution, due to the possible precipitation of solid urea at lower temperatures. Whilst a more diluted solution could be transported, such would bring about additional costs and efforts to transport water, and it would require additional energy input to increase the concentration before use. Therefore having a suitable urea powder as a starting material, would provide additional flexibility and cost saving in transportation.